Photodynamic therapy (PDT) is an innovative cancer treatment modality that is based on selective in vivo or ex vivo eradication of tumor cells through the action of localized photosensitizing agents. Continued advances in PDT technology and clinical applications will be facilitated by parallel advances in understanding of the biophysical and biochemical factors which modulate its effectiveness. Preliminary findings pertaining to the effector actions of iron and nitric oxide (NO) have important bearing on this issue and provide a strong impetus for this proposal: (i) Iron in the form of lipophilic chelates promotes photodynamic lipid peroxidation and cell killing when presented acutely to leukemia cells, but elicits a strong photoresistance when presented chronically and this is accompanied by ferritin induction; (ii) Iron-stimulated lipid peroxidation and phototoxicity are markedly suppressed by NO donors, particularly when NO release is photodynamically enhanced. Two hypotheses are advanced: (1) Iron status of tumors is an important factor in PDT responsiveness, acute iron exposure promoting cytotoxicity by amplifying lipid peroxidation and chronic exposure reversing this effect through the induction of ferritin. (2) NO generated by tumor vascular cells can either promote or suppress PDT cytotoxicity, depending on superoxide availability; suppression depends largely on NO's chain- breaking antioxidant effects. With an emphasis on the photoprotective effects of iron and NO, these hypotheses will be tested, using in vivo (rhabdomyosarcoma) and in vitro (leukemia, breast tumor, endothelial) models, two PDT sensitizers, and techniques such as high-performance liquid chromatography with electrochemical detection, electron spin resonance-spin trapping, immunoblotting, and band-shift electrophoresis. The plan is to investigate: (i) cellular susceptibility to photoperoxidative damage and photokilling in response to iron donors such as hemin and methemoglobin; (ii) effects of photoresistance-conferring iron stimuli on cellular iron levels and expression/activation of ferritin and other iron-responsive proteins; (iii) effects of photogenerated NO on membrane lipid peroxidation and cell killing; and (iv) effects of cell-generated NO on photodamage to vascular cells. This work is significant because it deals with novel forms of photoresistance elicited by iron and NO. A better understanding of these responses would have important bearing on the question of PDT efficacy and the design of rational clinical protocols involving PDT.